In recent years, various testing techniques using an electromagnetic wave whose frequency ranges from 30 GHz to 30 THz, which is a so-called terahertz wave, have been developed. Nondestructive testing using transparency and spectroscopy using that biomolecules have characteristic absorption in the terahertz band are the examples of the aforementioned testing techniques. An imaging device which obtains a tomographic image by analyzing a detected terahertz wave has been developed, too.
In the example described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-525447, assuming that an ultrashort pulsed laser beam is a pump light, an electromagnetic wave pulse having a frequency in the terahertz domain is generated and then a sample is irradiated with the electromagnetic wave pulse. The electromagnetic wave pulse which was reflected on or passed through the sample is detected by time-domain spectroscopy (TDS method) with the aforementioned pump light, a detector, and an optical delay system. In the TDS method in which the waveform itself of the electromagnetic wave pulse is observed, the amplitude and phase of an electric field are acquired at a time. In other words, the method is characterized by that information on both of the real part and the imaginary part of a complex index of refraction is able to be acquired at a time.
Meanwhile, with respect to a nondestructive testing method using visible to infrared light which is easy to deal with or a sound wave which is an elastic vibration wave (particularly, an ultrasonic wave having a frequency exceeding an audible range), conventionally, numerous methods have been developed and used. Like a tissue in a biological body, in the case of low contrast between a focused region and its surrounding, a method with a combination of light and an ultrasonic wave is used, too. In examples described in Japanese Patent Application Laid-Open No. 2007-216001 and Japanese Patent No. 4603100, a region of a sample required to be tested is irradiated with both of a laser beam and an ultrasonic wave. The ultrasonic wave modulates the density and the refractive index of the sample. Therefore, the intensity and the phase of the laser beam, which passed through or was reflected on the sample, are also affected by the ultrasonic wave. Laser beam variations, which occurred as a result, reflect the characteristics of the irradiated region. Therefore, the distribution of characteristics of the sample can be obtained by performing measurement while changing the position of the sample irradiated with the light and the ultrasonic wave.